Fluorinated polymers enjoy widespread use as hydrophobic, oleophobic coatings. These materials exhibit excellent environmental stability, high hydrophobicity, low surface energy and a low coefficient of friction, and are used in a number of applications ranging from nonstick frying pans to optical fiber cladding. Most fluoropolymers, however, are plastics that are difficult to process, difficult to apply and are unsuitable as coatings for flexible substrates due to their high rigidity. One example of a widely used fluorinated material is TEFLON™, a polytetrafluoroethylene. TEFLON™ is difficult to process in that it is a rigid solid that must be sintered and machined into its final configuration. Commercial application of TEFLON™ as a coating is complicated by its poor adhesion to substrates and its inability to form a continuous film. As TEFLON™ is insoluble, application of a TEFLON™ film involves spreading a thin film of powdered TEFLON™ onto the surface to be coated and, thereafter, the powdered TEFLON™ is sintered in place resulting in either an incomplete film or a film having many voids. As TEFLON™ is a hard inflexible plastic, a further limitation is that the substrate surface must be rigid, otherwise the TEFLON™ will either crack or peel off.
A limited number of commercial fluoropolymers, such as Viton, possess elastomeric properties. However, these materials have relatively high surface energies (as compared to TEFLON™), poor abrasion resistance and tear strength, and their glass transition temperatures are still high enough (greater than 0° C. for Viton) to significantly limit their use in low-temperature environments.
Accordingly, there is a need for fluoroelastomers having hydrophobic properties, surface energies and coefficients of friction at least equivalent to the fluorinated plastics (such as TEFLON™). Further, such fluoroelastomers must have high adhesion, high abrasion resistance and tear strength, low index of refraction and low glass transition temperatures so that they are suitable for any foreseeably low temperature environmental use. In addition, there is a need for fluoroelastomers that are easily produced in high yields and easy to use.
The most important criteria in the development of release (i.e., nonstick), high lubricity coatings is the minimization of the free surface energy of the coating. Free surface energy is a measure of the wettability of the coating and defines certain critical properties, such as hydrophobicity and adhesive characteristics of the material. For most polymeric surfaces, the surface energy can be expressed in terms of the critical surface tension of wetting ãc. For example, the surface energy of TEFLON™ (represented by ãc) is 18.5 ergs/cm2, whereas that of polyethylene is 31 ergs/cm2. Consequently, coatings derived from TEFLON™ are more hydrophobic and nonstick than those derived from polyethylene. A substantial amount of work has been done by the coating industry to develop coatings having surface energies lower than or comparable to TEFLON™, while at the same time exhibiting superior adhesion characteristics.
The literature teaches that in order to prepare coatings having the desirable low surface energy, the surface of the coating must be dominated by —CF3 groups. Groups such as —CF2—H and —CFH2 increase the surface energy of the material. The importance of the number of fluorine atoms in the terminal group (i.e., the group present on the surface) was demonstrated by Zisman, et al., J. Phys. Chem., 57:622 (1953); Zisman, et al., J. Colloid Sci., 58:236 (1954); and Pittman, et al., J. Polymer Sci., 6:1729 (1968). It was found that materials with terminal —CF3 groups exhibited surface energies in the neighborhood of 6 ergs/cm2, whereas similar materials with terminal —CF2H groups exhibited values in the neighborhood of 15 ergs/cm2, i.e., more than twice the value for the material with terminal —CF3 groups. TEFLON™ incorporates the fluorine moieties on the polymer backbone and does not contain pendant —CF3 groups. Consequently, TEFLON™ does not exhibit surface energies as low as polymers having terminal perfluorinated alkyl side chains.
A critical requirement in the production of an elastomer is that the elastomer have large zones, or “soft segments,” where little or no crosslinking occurs and where the polymer conformation is such that there is little or no compaction of the polymer as a result of crystallization. Intermediate of these soft zones are “hard blocks,” where there may be significant hydrogen bonding, crosslinking and compaction of the polymer. It is this alternating soft block and hard block that give the polymer its elastomeric properties. The longer the soft segment, the more elastic the elastomer.
Falk, et al. (U.S. Pat. No. 5,097,048) disclose the synthesis of bis-substituted oxetane monomers having perfluoro-terminated alkyl group side chains from bis-haloalkyl oxetanes, the glycols having perfluoro-terminated alkyl group side chains derived therefrom, including related thiol and amine linked glycols and dimer diols. Most of the fluorinated side chains are attached to the glycol unit by a thio, an amine or a sulfonamide linkage. Only a few examples describe glycols having perfluoro-terminated alkoxymethylene side chains; however, such glycols are crystalline materials.
Falk, et al. (EP 03 48 350) report that their process yields perfluoro-terminated alkoxymethylene neopentyl glycols composed of a mixture of (1) approximately 64% of a bis-substituted perfluoro-terminated alkyl neopentyl glycol, and (2) approximately 36% of a mono-substituted perfluoro-terminated alkyl neopentyl glycol product with a pendant chloromethyl group. Evidently, the mono-substituted product results from incomplete substitution of the second chloride on the bis-chloroalkyl oxetane starting material. Consequently, as noted from the Zisman and Pittman work described above, the presence of the —CH2Cl as a side chain significantly increases the surface energy of the coatings made from these polymers, thereby reducing the hydrophobicity and oleophobicity of the coatings.
Falk, et al. (U.S. Pat. No. 5,045,624) teaches preparation of dimers with fluorinated side chains having thio linkages, but not of dimers with fluorinated ether side chains. This is because the synthesis route used by Falk, et al. for preparing dimers with thio linkages cannot be used for the synthesis of dimers with ether linkages. In other words, Falk, et al. do not teach preparation of long chain polyethers with fluorinated ether side chains.
Falk, et al. (U.S. Pat. No. 4,898,981) teaches incorporation of their bis-substituted glycols into various foams and coatings to impart the desired hydrophobicity and oleophobicity. Classic polyurethane chemistry shows that while a plastic may form by reaction of Falk's glycols with diisocyantes, elastomers cannot form since there is no long chain soft segment. Such a soft segment is needed for the formation of an elastomer. Since the compounds of Falk, et al. are only one or two monomer units long, they are clearly too short to function as a soft segment for the formation of a polyurethane elastomer. In Falk, et al., the fluorinated glycol and isocyanate segments alternate, with the fluorinated glycol segments, being nearly the same size as the isocyanate segments. It is well known to those of skill in the art that such a polymer structure will not yield elastomers.
None of the Falk, et al. references teach or show a homoprepolymer or coprepolymer made from bis-perfluoro-terminated alkoxymethylene oxetanes, nor polyurethanes derived therefrom or from the corresponding glycols. All of their polyurethanes are made directly from the thiol-linked monomers and dimers and not via a prepolymer intermediate. In the examples provided in Falk, et al. (U.S. Pat. No. 5,045,624), particularly where the perfluoro-terminated side chains are large and for all of the dimers, all have thiol linkages, i.e., no ether side chains are shown. The polyurethanes disclosed by Falk, et al. (U.S. Pat. No. 4,898,981) are made from the perfluoro-terminated alkylthio neopentyl glycol. However, Falk, et al. (U.S. Pat. No. 5,097,048) in Example 12, show a polyether prepolymer prepared from a bis-substituted perfluoroalkylthio oxetane. The prepolymer obtained was a white waxy solid, clearly not an elastomer. No characterization as to the nature of the end groups, polydispersity, equivalent weights, etc. of the waxy solid was given. Absent such a characterization, it is unknown whether the material of Falk, et al. may be further reacted with an isocyanate to produce a polyurethane polymer. No examples of the preparation of a polymer from any prepolymer is given.
Vakhlamova (Chem. Abst. 89:110440p) teaches the synthesis of oxetane compounds substituted at the number 3 carbon of the oxetane with —CH2O—CH2—CF2—CF2—H groups. The terminal alkyl portion of this substituent is thus: —CF2CF2—H, wherein the terminal or omega carbon bears a hydrogen atom. As discussed above, the Zisman and Pittman work shows that the presence of the hydrogen significantly increases the surface energy of the polymer derived from these monomers. Falk, et al. (U.S. Pat. No. 5,097,048) also recognizes that surface energy increases with the hydrogen atom on the terminal carbon by stating that “fluoroalkyl compounds which are terminally branched or contain omega-hydrogen atoms do not exhibit efficient oil repellency.” Further, Vakhlamova focuses on the bis-substituted monomer as he hydrolyzes the monomer and then polymerizes the resultant monomeric glycol.
A characteristic of the polymers formed from the polymerization of the bis-substituted oxetanes of Falk, et al., and the other proponents of bis-substituted oxetanes is that the resulting products are crystalline solids. The bis-side chains are highly ordered and symmetric. Consequently, they pack efficiently to form a crystalline structure. For example, a prepolymer prepared from 3,3-bis-(chloromethyl)oxetane is a crystalline solid that melts in the neighborhood of 220° C. This significantly affects the commercial use of these polymers since either or both mixing and elevated temperatures will be required in order to dissolve or melt the Falk, et al. polymer for further polymerization or application.
As such, to date, the polymerization of the bis-substituted perfluorinated alkoxymethylene oxetanes has not resulted in useful materials. The polymers derived from the bis-substituted perfluoroalkylthiol oxetanes are waxy solids and will not function as a soft segment in the preparation of commercially useful elastomers and coatings. Further, the ability of a bis-substituted oxetane monomer to homopolymerize appears to be dependent upon the nature of the side chain at the 3-carbon with no assurance that polymerization will occur, the difficulty of polymerization apparently being due to the steric interference by the side chains. Polymerization, and the products of polymerization, of the bis-substituted monomer accordingly are unpredictable and inconsistent.
U.S. Pat. Nos. 5,654,450, 5,668,251, 5,650,483, 5,668,250 and 5,703,194, all of which have issued to Malik, et al., disclose fluorinated elastomers and a production strategy therefor, beginning with a premonomer production process that is easy and inexpensive, to produce an asymmetrical mono-haloalkyl methyl oxetane premonomer, which upon further reaction produces an oxetane monomer having a single fluorinated side chain, which mono-substituted fluorinated monomer is capable of homopolymerization and copolymerization to produce an essentially non-crosslinked soft segment, difunctional, linear, asymmetric prepolymer for further reaction to produce fluorinated elastomers and thermoset plastics, resins and coatings having hydrophobic properties, low surface energy, very low glass transition temperatures, low dielectric constants, high abrasion resistance and tear strength, high adhesion and low refractive indices.
The contributions of U.S. Pat. Nos. 5,654,450, 5,668,251, 5,650,483, 5,668,250 and 5,703,194 have resulted in very useful fluorinated elastomers and methods for their preparation. However, it would be advantageous if bis-substituted fluorinated oxetane monomers could be homopolymerized and copolymerized to produce essentially non-crosslinked, difunctional, linear, asymmetric prepolymers that, in turn, could be further reacted to produce fluorinated elastomers and thermoset plastics, resins and coatings having useful properties, a goal which researchers have not yet achieved despite great efforts in this area. The present invention achieves this and other goals.